Electrolytic anode

ABSTRACT

AN IMPROVED ANODE FOR THE ELECTROLYSS OF BRINES IS COMPRISED OF A CORROSION RESISTANT VALVE METAL SUBSTRATE, A THIN POROUS ADHERENT EXTERIOR COATING OF SILICA, AND BETWEEN THE SUBSTRATE AND EXTERIOR COATING A THIN LAYER OF RUTHENIUM OXIDE.

United States Patent 3,654,121 ELECTROLYTIC ANODE Carl D. Keith, Summit,Alfred J. Haley, Jr., Florham Park, and Robert M. Kero, Cranford, N.J.,assignors to Engelhard Minerals & Chemicals Corporation, Newark, NJ.

No Drawing. Filed Dec. 23, 1968, Ser. No. 786,438 The portion of theterm of the patent subsequent to Dec. 28, 1988, has been disclaimed Int.Cl. B01k 3/04 U.S. 204-290 F 3 Claims ABSTRACT OF THE DISCLOSURE Animproved anode for the electrolysis of brines is comprised of acorrosion resistant valve metal substrate, a thin porous adherentexterior coating of silica, and between the substrate and exteriorcoating a thin layer of ruthenium oxide.

This invention relates to novel anodes for cells used for theelectrolysis of brines, and more particularly to improved anodescomprised of platinum group metal coated electrolytic valve metals and amethod for obtaining such anodes.

The anodes of the present invention are particularly useful in cellsused for the production of chlorine and caustic soda by the electrolysisof an aqueous solution of sodium chloride. In such cells graphite anodesare usually used commercially. Although the graphite anodes are notentirely satisfactory because their wear rates are high and impuritiessuch as CO are introduced in the products, no satisfactory substituteshave yet been found.

Platinum group metal coated electrolytic valve metals have been proposedas substitutes for graphite anodes. These metallic anodes offer severalpotential advantages over the conventional graphite anodes, for example,lower overvoltage, lower erosion rates, and higher purity products. Theeconomic advantages gained from such anodes, however, must besufliciently high to overcome the high cost of these metallic anodes.Anodes proposed heretorfore have not satisfied this condition. Thereforecommercialization of the platinum group metal anodes has been limited.

One problem is the life of the metallic anodes. A factor whichcontributes to shortening the anode life is the soealled undercuttingelfect. For economic reasons the precious metal coatings are very thinfilms so that exposure of the substrate is imminent. This isparticularly true in the use of low overvoltage coatings which areinherently porous. Although corrosion resistant, the valve metals areattacked through the pores of these coatings thereby shortening the lifeof the anodes.

Another problem is the loss of precious metal during operation of thecell. Although the loss is gradual, it is costly because the preciousmetals are expensive and because the erosion of the thin coatingshortens the anode life. The loss of precious metal may be frommechanical wear. At the high current densities desirable in commercialinstallations, the increased rate of flow and the excessive gassing isconducive to such mechanical wear. In mercury cells a contributingfactor is amalgamation of the precious metals.

A further problem in mercury cells is shorting of the cell on contact ofthe precious metal 'with the mercury with consequent elfects, such asamalgamation, change in the surface of the anodes with resultant harmfulchange in electrolytic properties, and cell stoppage.

A still further consideration which is of major importance in the highlycompetitive manufacturing processes involving the electrolysis of brinesis the power consump- 3,654,121 Patented Apr. 4, 1972 tion associatedwith the anodes. Power costs represent a substantial percentage of thetotal production costs and even a small reduction in power consumptionproduces a material economic advantage.

It was an object of this invention to provide metallic anodes withimproved physical and electrical characteristics. It was a furtherobject to provide a process for the electrolysis of brines which can beeffected with materially lower production costs.

In accordance with this invention the electrolysis of brines can beeffected with a materially lower power consumption. This is achieved bythe use of an improved anode. The anode not only reduces the powerconsumption in the cell, but also it has been found to have long lifeand low metal losses due to mechanical wear and amalgamation. Theresistance to amalgamation makes the anode particularly useful inmercury cells.

The anode of the present invention is comprised of a corrosion resistantmetal substrate, a ruthenium oxide coating, and a thin porous adherentcoating of silica over the ruthenium oxide. The silica coating has ahigh surface area, typically of at least about 30 square meters per gram(m. /g.).

The corrosion resistant metal substrates, the so-called valve metals,used for electrolytic anodes are well known in the field. They are muchless expensive than platinum group metals and they have properties whichrender them corrosion resistant to the anodic environments inelectrolysis cells. Examples of suitable corrosion resistant valvemetals are Ti, Ta, Nb, Hf, Zr, W, Al, and alloys thereof. It is alsowell known to have the valve metal as a layer on a base metal such ascopper which is a good conductor but corrosive to the environment, andsuch modifications are within the scope of this invention.

The silica coating not only minimizes the contact of the precious metallayer with the electrolyte, but also minimizes penetration of theelectrolyte to the valve metal and thus limits the extent ofundercutting effects. An other advantage is that it minimizes shortingand the concomitant problems. Surprisingly however, despite thedielectric characteristics of the exterior coating, these advantages aregained without sacrificing the desirable electrical properties of theprecious metal anodes. Indeed, the exterior porous high surface areasilica coating improves the electrolytic properties of the thin preciousmetal coatings. A still further advantage of the anodes of thisinvention is that the high surface area porous exterior coating isconducive to gas evolution.

The anodes of this invention are prepared by first forming a rutheniumoxide layer on the base metal substrate and then depositing a silicacoating on the ruthenium oxide.

Many methods are known for forming adherent ruthenium oxide films on ametal substrate. For example, after etching and cleaning the surface ofthe base metal, ruthenium metal or a ruthenium salt is deposited on thesubstrate and then the coated substrate is subjected to elevatedtemperature in an oxidizing atmosphere. The ruthenium metal or salt isdeposited in a variety of well known ways, e.g. the ruthenium metal maybe deposited as a finely divided dispersion in an organic vehicle or byplating, sputtering, vacuum deposition, the ruthenium salt may bedeposited by applying such salt dispersed or dissolved in an organic oraqueous medium. The conversion to the oxide is then effected by firingthe coating in an oxygen-containing atmosphere, e.g. air, preferably inthe temperature range of about 400 to 800 C. The firing time depends onthe temperature, oxidizing atmosphere uesd, and the thickness of theruthenium metal coating applied. Typically a suitable ruthenium oxide isformed by firing the metal film in air at 500 C. for about five minutes.

The exterior high surface area porous silica coating is deposited from adispersion or solution containing hydrophilic silica or a silicacompound precursor in very fine particle size, and the silica coating isfired at temperatures greater than about 400 C. to promote bonding. Whenfired at temperatures lower than about 400 C. the coatings are notsufiiciently adherent. A preferred method is to deposit the silica froman aqueous colloidal silica solution. Preferred temperatures for formingan adherent porous coating are 400 to 800 C. Coatings formed in thismanner are adherent and porous and have a high surface area. More thanone coating of silica may be applied. Generally, the silica coatings areeffective at a thickness of up to about 200 microinches. Thickercoatings are often not sutficiently porous. Alternatively, multiple thincoatings may be formed by depositing alternate layers of ruthenium oxideand silica, thereby forming a hard durable multilayer coating on thesubstrate. Although the multilayer coatings are effective at thicknessof over 200 microinches, there is no advantage in forming thickercoatings because of their durability even when exceedingly thin.

The following examples are given by way of illustration and not as alimitation of the invention. It will be appreciated that modificationswithin the scope and spirit of the invention will occur to those skilledin the art.

Examples 1, 2 and 3 show comparative tests in diaphragm and mercuryelectrolysis cells using various anodes. For each anode a sheet ofcommercially pure titanium, /2" x 3 x 0.063", is prepared for coating byetching in concentrated hydrochloric acid for a period of 18 hours atroom temprature and cleaning in fiuoboric acid.

RuO coatings are prepared as follows:

An aqueous solution of RuCl (containing 10.35% by Weight of Ru) isapplied to one side of a titanium sheet using a brush. Successive coatsare applied, each being fired at 500 C. in air for five minutes until acoating of the desired thickness is obtained. Alternatively a rutheniumresinate solution (containing 4% by weight Ru) is applied. In stillanother alternative method an alcohol based paint is used. This paint iscomposed of l g. of RuCl;,, 1 ml. of linalool and 30 ml. of 2-propanol.X-ray dilfraction analysis of samples similarly prepared, by firing thedeposited coating in air at the indicated conditions, showed that amajor portion of the ruthenium was converted to ruthenium oxide.

Porous adherent silica coatings are prepared as follows:

After forming the RuO layer, it is overcoated with S by applying aformulation containing hydrophilic colloidal silica. Ludox HS, anaqueous collodial silica solution, is used in the formulation. Theformulations contain about 10% colloidal silica and 90% water. Filmforming additives such as sodium titanate, silicate or borate may beincorporated in minor amounts in the colloidal silica solution. Forexample, suitbale coatings are made from a formulation composed of 10%colloidal silica, 0.5% sodium titanate and 85.5% water. Successive coatsof silica are applied and fired in air at 500 C. for 5 minutes until acoating of the desired thickness is obtained.

The thickness of the coatings is determined gravimetrically.

EXAMPLE 1 Two samples are prepared having a RuO coating equivalent to 17microinches of Ru metal on a titanium substrate.

Sample B is used as prepared.

Sample A is overcoated with 100 microinches of SiO using the methoddescribed above. The silica has a surface area of about 70 mF/g.

Sample A and Sample B are used as anodes in a laboratory scale diaphragmcell for the electrolysis of NaCl solution. The tests are run at atemperature of 4 C. and a current density of 1000 amperes per squarefoot (ASF). The chlorine overvoltage is determined with a conventionalLuggin capillary probe, and the results are set forth in Table I.

Cell potential after 210 hours at 1,000 a.s.f. (volts) 3.

l E1102 and SiO: overcoating.

2 After 210 hours, Sample B would not draw the specified current density at its initial cell potential. Upon raising the cell potentialrapid disintegration of both the coating and the substrate resulted.

This example demonstrates the superior electrical and wear properties ofthe anode having the SiO exterior coating of this invention over ananode having a RuO layer and no overcoating of silica.

EXAMPLE 2 Samples similar to those described in Example 1 are prepared.Sample C is a titanium substrate with a RuO coating having a thicknessequivalent to 17 microinches of Ru. Sample D is a titanium substratewith a RuO layer equivalent to 17 microinches of Ru and microinchesovercoating of silica. Each of the samples is masked with pressure tapeso that an area of 0.049 in. of coating remains exposed. Samples C and Dare then used as anodes in a small cell using a mercury pool as thecathode and a 25% NaCl solution as the electrolyte. The anodes aresubjected to a mercury shorting test as follows:

The exposure area of the test coating is allowed to generate chlorine at1000 ASF in the brine and then it is submerged in the mercury pool andthe change in current density is measured. The tests show that Sample D,having the Ru0 layer and the SiO exterior coating is very much lesssusceptible to shorting than the same coating without the protectiveexterior coating of SiO It will be appreciated that since the resistanceto shorting is higher the anodes of this invention may be positioned incloser spacial relationship with a mercury cathode without danger ofshorting and with concomitant lower power requirements.

EXAMPLE 3 Samples similar to those described in Example 1 are prepared,except that the RuO layer is thinner. Two samples are prepared eachhaving a Ru0 coating equivalent to 2 microinches and Ru on a titaniumsubstrate.

Sample E is used as prepared.

Sample F is overcoated with microinches of SiO using the methoddescribed above.

Samples E and F are used as anodes in a laboratory scale diaphragm celland tested for chlorine overvoltage using the procedure described inExample 1. The cell using Sample F, the anode in accordance with thisinvention, has an initial chlorine overvoltage of 220 millivolts and acell potential of 4.30 volts. The cell using Sample E as the anode showserratic behavior. The coating of Sample E is poorly adherent and theerratic results are believed to be attributable to this poor adherenceof the coating and also to the insufficient protection provided by theRuO coating of this degree of thinness.

This test demonstrates the improved physical and electrical propertiesof anodes of this invention. Such improvements not only permit operationof a cell with lower power requirements but also demonstrates theimproved life of the anodes since they are operable with thinnercoatings of precious metal than anodes without such coatings.

EXAMPLE 4 Two sheets of commercially pure titanium /2 x 3" x 0.063", areprepared for coating by sandblasting the surfaces with aluminum oxidegrit followed by cleaning with an abrasive cleanser. Both sheets arethen coated on both sides with a formulation composed of (by weight)11.5% ruthenium chloride, 42.3% 2-propanol, and 46.2% linalool. Thecoated substrates are heated to 300 to 400 C. for l to 2 minutes andthen fired at 500 C. for 5 minutes in an open air furnace to form a RuOcoating.

Sample G is prepared by repeating the application of the rutheniumformulation and heat treatment twice, so that a total of three coats ofruthenium oxide are applied.

Sample H is prepared by overcoating the first ruthenium oxide coatingwith a porous silica coating. The porous silica coating is formed byapplying an aqueous colloidal silica solution composed of (by weight)31.6% Ludox HS (containing 30% SiO 0.5% sodium titanate powder, and67.9% water. The silica-coated substrate is heated to 500 C. for 5minutes. Thereafter the procedure of applying and firing alternatecoatings of ruthenium oxide and silica is repeated twice.

The composition of the samples is as follows:

Coating Sample G Sample H R1102, percent. 100 43. 3 Si02, percent 0 56.7

scale diaphragm cell and tested for chlorine overvoltage using theprocedure described in Example 1. Sample G having 3 coatings ofruthenium oxide has an initial chlorine overvoltage of 155 millivoltsand a cell potential of 4.20 volts. Sample H, a multilayer RuO SiOcoating prepared in accordance with the present invention, has aninitial chlorine overvoltage of 10 millivolts and a cell potential of4.30 volts. In addition the multilayer R SiO coating, applied inalternate layers, is more adherent than the R110 coating of Sample G.

This example not only illustrates a method of preparing the RuO- and SiOcoating by depositing alternate layers of Ru0 and SiO but also furtherdemonstrates the improved physical and electrical properties of anodesof this invention.

We claim:

1. An electrolytic anode comprising a corrosion resistant valve metalsubstrate, a thin adherent porous exterior coating of silica, andbetween the substrate and exterior coating a thin layer of rutheniumoxide.

2. An electrolytic anode of claim 1 wherein the exterior silica coatinghas a thickness of up to about 200 microinches.

3. An electrolytic anode of claim 1 wherein the exterior silica coatinghas a surface area of at least 30 m. /g.

References Cited FOREIGN PATENTS 6,606,302 11/ 1966 Netherlands 204290DANIEL E. WYMAN, Primary Examiner I. VAUGHN, Assistant Examiner

